Allows disabling all containment actions, such as context menus, middle click paste, wheel desktop switch, etc.

This key used to be there in 4.x but didn't really work. There also used to be a dedicated one for containment context menus but I think this distinction doesn't make a lot of sense, except maybe for disabling the context menu but still allowing virtual desktop switching on wheel?!
But then, the old docs weren't particularly clear on what "containment context menu" restricts. The comment implies it is only the containment context menu (desktop menu) while keeping applet context menus in tact? That would make it a lot more complicated to implement, ie. don't add containment actions to applet context menus etc etc.
And Harald just wants a way to show a minimalist Plasma with panels that the user cannot mess with in any way, so that patch does that.

Added the following in kdeglobals

[KDE Action Restrictions]
plasma/containment_actions=false

no longer could right click panel or desktop (system tray does its own thing and needs separate fixing).
Verified that it doesn't actually change the config file, so when the restriction is lifted, plasma loads the previously configured actions again. Also speeds up plasma startup a bit as it doesn't bother loading all the action plugins.